1. Field of the Invention
The present invention relates to an art for appropriately subjecting an optical disc including a recording layer employing organic dye to data recording.
2. Description of the Related Art
For example, an optical disc for recording to which the Blu-ray standard (hereafter, abbreviated as “BD standard”) has been applied includes a disc-shaped substrate having spiral-shaped guide grooves (also referred to as pregrooves) on one face, a reflecting layer formed on one face of the substrate, a recording layer formed on the reflecting layer, and an optically-transparent cover layer formed on the recording layer. With such an optical disc, a recording laser beam is irradiated onto the recording layer on the pregrooves, i.e., onto the grooves to form a mark, thereby recording data. The recording laser beam is irradiated with a multi-pulse-type recording pulse (write pulse) schematically illustrated in FIG. 3A, or with a single-pulse-type recording pulse in some cases. With regard to this recording pulse, space formation power Ps which is recording power necessary for space formation is employed at the time of space formation, and main power Pw is employed at the time of mark formation. Bias power Pbw is employed between the pulses of the top pulse and thereafter with a mark of 3 T or more, cooling pulse power Pc is employed after the final pulse of the mark. Here, with the recording pulse according to FIG. 3A, with formation of a mark of 3 T or more, in order to prevent disorder on the shape of the mark from occurring due to the accumulation of heat generated by the laser irradiation corresponding to each pulse, the bias power Pbw is set smaller than the space formation power Ps. Of such optical discs, with regard to an optical disc including a recording layer employing an inorganic material, upon recording being performed with established recording setting conditions, i.e., strategy, properties such as shown in FIG. 1 are obtained, for example. Specifically, the horizontal axis represents the main power (recording power at the time of mark formation) Pw, and the vertical axis on the left side represents DC jitter (%) (hereafter, abbreviated as DCJ), i.e., the level of positional and width irregularities when recording data is converted into a recording signal, and the vertical axis on the right side represents an asymmetry value (hereafter, abbreviated as “Asym” in the drawings). Note that DCJ is the standard deviation of the phase difference between a binary RF signal binarized from a RF signal and a clock signal generated from the binarized RF signal thereof. Also, the asymmetry value is an evaluation index representing the symmetry between the shortest recording mark and shortest space and the longest recording mark and longest space of a RF signal. Note that a β value may be employed instead of the asymmetry value. With the GND level as reference wherein AC coupling, i.e., the DC components of the RF signal are removed, if we say that the difference between the amplitude and GND level of the longest recording mark M8T is a, and the difference between the amplitude and GND level of the longest space S8T is b, the β value is a numeric value (hereafter, referred to as “β value”) computed with β=(a−b)/(a+b).
As shown in FIG. 1, DCJ represents a curve such as a quadratic function wherein the DCJ becomes the minimum at a Pw of around 5.4 mW, and the asymmetry value increases generally in a linear manner as the main power Pw increases. With regard to an optical disc employing an inorganic material, in a case wherein there is a problem when detecting the asymmetry value or β value, the main power Pw is adjusted in accordance with the curve such as shown in FIG. 1, whereby a desired asymmetry value or β value can be obtained. Accordingly, for example, with working optimum power control (hereafter, abbreviated as WOPC) during operation or the like, control of the main power PW is performed with the asymmetry value as an evaluation index, whereby irregularities relating to a disc itself such as the film thickness, reflecting film thickness, plate thickness, warping, and so forth of a recording material, or irregularities relating to change in temperature of a laser diode, a drive such as servo operation, or the like, can be handled.
On the other hand, with regard to an optical disc including a recording layer employing organic dye, upon recording being performed with established strategy or the like, properties such as shown in FIG. 2 are obtained, for example. The horizontal axis and vertical axes are similar to those in FIG. 1. In the case of organic dye as well, DCJ represents a curve such as a generally quadratic function in the same way as with the case of an inorganic material though the values are different, but with regard to the asymmetry value, we cannot find linearity such as shown in FIG. 1 in the case of an inorganic material. That is to say, with the asymmetry value and β value, this state is a state wherein determination cannot be made whether or not the appropriate main power Pw is set.
Note that with Japanese Unexamined Patent Application Publication No. 2003-323717, technology has been disclosed wherein, in order to improve recording precision while suppressing influence of heat accumulation and thermal diffusion on the recording face of an optical information recording medium with an optical information recording device for irradiating a recording pulse onto a recording information recording medium, when a recording mark is formed on the optical information recording medium to record information, the power of a recording pulse is changed between write power Pw and bias power Pbw necessary for forming a recording mark, thereby decreasing the bias power Pbw as compared to read power Pr necessary for playing the recording mark. Note however, this technology does not enable the above-mentioned problem to be solved.
As described above, when employing an optical disc employing organic dye, employment of established write strategy has a problem in that various irregularities such as described above cannot be handled with the asymmetry value or β value as an evaluation index.
Further, in a case wherein the detection precision of the asymmetry value or β value is low, there is a need to increase the tilt of a straight line representing the relation between the asymmetry value or β value and the main power Pw. Also, there is a need to maintain the linearity of the straight line representing the relation between the asymmetry value or β value and the main power Pw.